text: Following a similar logic as above, one can derive the kinetic model for mass diffusivity of a dilute gas:
  Consider a steady diffusion between two regions of the same gas with perfectly flat and parallel boundaries separated by a layer of the same gas. Both regions have uniform number densities, but the upper region has a higher number density than the lower region. In the steady state, the number density at any point is constant (that is, independent of time). However, the number density  in the layer increases uniformly with distance  above the lower plate. The non-equilibrium molecular flow is superimposed on a Maxwell-Boltzmann equilibrium distribution of molecular motions.
  Let  be the number density of the gas at an imaginary horizontal surface inside the layer. The number of molecules arriving at an area  on one side of the gas layer, with speed  at angle  from the normal, in time interval  is 
  These molecules made their last collision at a distance  above and below the gas layer, where the local number density is
  Again, plus sign applies to molecules from above, and minus sign below. Note that the number density gradient  can be considered to be constant over a distance of mean free path.
  Integrating over all appropriate velocities within the constraint
  yields the molecular transfer per unit time per unit area (also known as diffusion flux):
  Note that the molecular transfer from above is in the  direction, and therefore the overall minus sign in the equation. The net diffusion flux across the imaginary surface is thus
  Combining the above kinetic equation with Fick's first law of diffusion
  gives the equation for mass diffusivity, which is usually denoted  when it is a dilute gas:
  The corresponding expression obtained from Revised Enskog Theory may be written as
  where  is a factor that tends to unity in the limit of infinite dilution, which accounts for excluded volume and the variation chemical potentials with density.